Update, 11:25 a.m.: Andrea has been downgraded to a subtropical depression and the National Hurricane Center writes in its 11 a.m. advisory that it is expected to dissipate soon.
Original post from 10:25 a.m.
On Monday afternoon, a low pressure system south of Bermuda was upgraded to Subtropical Storm Andrea, the first of the 2019 hurricane season; 11 days before the official beginning of the season.
This makes 2019 the fifth consecutive year that a named storm has formed before the official start of Atlantic hurricane season, which is June 1.
Andrea has a very brief window of time to exist before being absorbed by an approaching cold front. Increasing wind shear — which is disruptive to the storm’s circulation — nearby dry air and low ocean temperatures will combine to dissipate this storm within a couple of days.
The forecast from the National Hurricane Center is for Andrea to turn to the northeast on Tuesday, which will bring it close to Bermuda. The effect there will be minimal, though — with just some breezy rain showers possible Wednesday.
Andrea in historical context
Andrea, which formed on May 20, becomes the sixth preseason storm to develop in the past 10 years. It follows Alberto in 2012 (May 19), Beryl in 2012 (May 25), Ana in 2015 (May 8), Bonnie in 2016 (May 28), Arlene in 2017 (April 20) and Alberto in 2018 (May 26).
This list does not include Alex, which formed on Jan. 16, 2016. While falling in the 2016 calendar year, Alex was meteorologically a remnant of the 2015 season. But if you wish to include it in your list, then we’ve had seven preseason storms in the past decade.
There is clearly a trend toward earlier instances of first storm formation over the past five decades, with a range spanning from April 20 to Aug. 30. Still, the median date over this period is June 20.
Clearly, nothing is magical about the official June 1 start of hurricane season. The official start and end dates were never intended to contain all of the activity, just the vast majority of it.
When an official “hurricane season” was first defined about 85 years ago, it spanned June 15 to Oct. 31, then it was adjusted to June 15 through Nov. 15, then June 1 through Nov. 15, and in 1965 to its current June 1 through Nov. 30. It could certainly be adjusted again. If it is expanded to May 15 through Nov. 30, it would conveniently match the East Pacific hurricane season.
What does “subtropical” storm mean?
You may have noticed that Andrea has been named a subtropical storm rather than a tropical storm. What’s the difference? In terms of effects experienced on the ground, not a whole lot. It’s a technical distinction determined by the structure of the storm and where it is born. A subtropical storm is essentially a mix of a tropical storm — which forms at low latitudes — and an extratropical storm — which forms at high latitudes.
The National Hurricane Center has been tracking subtropical cyclones for many decades, but began assigning names to them in 2002, and all historical statistics have since been revised to incorporate them. The map below may help you visualize the zones in which you’d be likely to find them. Note that there are not strict boundaries, and the zones overlap by quite a lot.